EUV light, e.g., electromagnetic radiation in the EUV spectrum (i.e. having wavelengths of about 5-100 nm), may be useful in photolithography processes to produce extremely small features, e.g., sub-32 nm features, in semiconductor substrates, such as silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements, e.g., xenon, lithium or tin, indium, antimony, tellurium, aluminum, etc., with one or more emission line(s) in the EUV spectrum. In one such method, often termed laser produced plasma (“LPP”), a plasma can be produced by irradiating a target material, such as a droplet, stream, or cluster of material having the line-emitting element, with a laser beam. Another method involves disposing the line-emitting element between two electrodes. In this method, often termed discharge produced plasma (“DPP”), a plasma can be produced by creating an electrical discharge between the electrodes.
For these processes, the plasma is typically produced in a sealed vessel, e.g., vacuum chamber, and monitored using various types of metrology equipment. A typical EUV light source may also include one or more EUV mirrors. In general, these EUV mirrors may be either grazing incidence-type mirrors, or near-normal incidence type mirrors. e.g., a substrate covered with a multi-layer coating such as Mo/Si. One or more of these mirrors may then be disposed in the sealed vessel, distanced from the irradiation site, and oriented to direct EUV light emitted from the plasma to an EUV light source output location. By way of example, for an LPP setup, the mirror may be in the form of e.g., a prolate spheroid having a circular cross-section normal to a line passing through its foci, and having an elliptical cross-section in planes, including the line passing through the foci. In some cases, an aperture may be provided to allow the laser light to pass through and reach the irradiation site. With this arrangement, the irradiation site may be positioned at or near a first focus of the prolate spheroid, and the light source output may be positioned at, near or downstream of the second focus.
Several factors may be considered when selecting a substrate material for an EUV mirror. These can include temperature stability, vacuum compatibility, chemical stability, manufacturability, including the ability of the material to be easily shaped and polished, thermal mass, and material availability and cost. With these factors in mind, substrate candidates can include silicon (single crystal and polycrystalline) and silicon carbide.
In addition to generating EUV radiation, these plasma processes described above may also generate undesirable by-products, so-called debris, in the plasma chamber which can include high energy ions and/or atoms including target material vapor and/or clumps/micro-droplets of the target material. These plasma formation by-products can potentially heat, damage or reduce the operational efficiency of the various plasma chamber optical elements including, but not limited to, the mirrors described above, the surfaces of metrology detectors, windows used to image the plasma formation process, and the input window allowing the laser to enter the plasma chamber. The debris may be damaging to the optical elements in a number of ways, including coating them with materials which reduce light transmission, penetrating into them and, e.g., damaging structural integrity and/or optical properties, e.g., the ability of a mirror to reflect light at such short wavelengths, corroding, roughening or eroding them and/or diffusing into them.
Accessing contaminated or damaged optical elements in the plasma chamber for the purpose of cleaning or replacing the elements can be expensive, labor intensive and time-consuming. In particular, these systems typically require a rather complicated and time-consuming purging and vacuum pump-down of the plasma chamber prior to a re-start after the plasma chamber has been opened. This lengthy process can adversely affect production schedules and decrease the overall efficiency of light sources for which it is typically desirable to operate with little or no downtime.
For some target materials, e.g., tin, it may be desirable to introduce an etchant, e.g., Cl2, Br2, HBr, HI, HCl, H2, CF3, H radicals, some other halogen-containing compound, or combinations thereof, into the plasma chamber to etch material, e.g. debris that has deposited on the optical elements. This etchant may be present during light source operation, during periods of non-operation, or both. To increase the efficacy of these etchants, it may be desirable to heat and/or maintain the affected surfaces within a preselected temperature range to initiate reaction and/or increase the chemical reaction rate of the etchant and/or to maintain the etching rate at a certain level. For other target materials, e.g., lithium, it may be desirable to heat the affected surfaces where lithium debris has deposited to a temperature sufficient vaporize at least a portion of the deposited material, e.g., a temperature in the range of about 400 to 550 degrees C. to vaporize Li from the surface, with or without the use of an etchant.
Depending on the light source configuration, the above-described heating may be applied during EUV light source operation (i.e., while a plasma is being generated) and/or during startup, e.g., until the optic receives sufficient heat from another source such as the plasma and/or during periods of EUV light source downtime.
With the above in mind, applicants disclose systems and methods for heating an EUV collector mirror.